This invention relates to improvements in vapor/liquid contact apparatus and more particularly to an improved sealing vapor/liquid contact tray downcomer construction for absorption and similar towers or columns.
A major concern in commencing operation of a tray-containing vapor/liquid contacting apparatus such as an absorption or a distillation column is the sometimes difficult problem of establishing a liquid seal in the downcomers.
In conventional operation of a countercurrent vapor/liquid contacting tray (also referred to herein as a deck) vapor flows upwardly through tray perforations and liquid flows through the downcomer. The liquid seal at the bottom of the downcomer keeps vapor from entering the downcomer and the velocity of the vapor keeps most of the liquid from weeping through the tray perforations.
During startup of the apparatus, a reverse situation occurs. Until a downcomer seal is established, vapor flows upwardly through the downcomer as well as the tray perforations, and at low vapor velocity the liquid pours through the perforations rather than flow across the tray and outlet weirs to reach the downcomers. As vapor and liquid rates are increased the situation may occur that, before the liquid rate is high enough to seal the downcomer, the vapor flow up the downcomer is sufficient to prevent liquid downflow. In the current art, the start-up procedure must be changed or the trays/downcomers modified to avoid this problem.
When designing a vapor/liquid contactor, the traditional way to widen the operating conditions over which downcomer sealing and successful column startup can occur is to use smaller clearance area under the downcomers; however, the need to accommodate high flow rates in operation may make it impossible to reduce the clearance area enough to avoid a sealing problem on startup. Further, low downcomer clearance in a seal pan (typically on the bottom tray downcomer) runs the risk of plugging caused by deposition of solids, which solids may be present in or formed during processing of the contact fluids.
Another traditional way of promoting successful contactor startup is to incorporate recessed inlet pans; however, this is a relatively costly solution. Moreover, like the low downcomer clearance there remains a possibility of plugging. Further, inlet pans have the additional disadvantage of a lower separation efficiency, especially in small columns, since the flow through the requisite drain holes bypasses two column trays.
The present invention provides a novel method and apparatus to seal vapor/liquid contacting trays on startup.